Transcript Old Coins - University of Queensland

Faces and Fakes: Ancient and
Modern coins - Summary of a
CyberSTEM presentation
We will be looking at a range of modern and
ancient coins
We will talk about:
1) Scanning electron microscopy
2) Sample preparation
3) Chemical analysis of metals and oxidation
4) Modern, ancient and fake coins, their value
and identification
Text and images by the Centre for Microscopy & Microanalysis,
University of Queensland, Australia, August 2007
Coins are made out of
metal. What metals are used
and why?
Below
Australian coin
Above
Roman coin (from London):
Constantine I
Coins were originally worth their weight in the
metal from which they were struck or cast
But not all coins are pure metal
Bronze is a copper alloy (copper plus other
metals such as tin and other elements)
Some coins are precious metal mixed with metal
of lesser value and others are plated, that is one
metal over another of lesser value.
Useful references: http://en.wikipedia.org/wiki/Bronze
http://en.wikipedia.org/wiki/Coins
What are modern coins made of?
How can we analyse the elements
present in a coin?
Is modern money worth the value
of the metal that makes it?
What makes an ancient coin
precious? What can they tell us?
Sample preparation
Coins ready to go into a scanning electron microscope
Scanning electron microscopy (SEM) can be used
to look at the surface features of coins
This can tell us about history – in the case of
ancient coins since details were changed regularly
The X-rays produced when electrons interact with
the sample can tell us about the surface
composition of coins
[for details on science and background of technique see the
PowerPoint presentation called Background theory and
terminology]
We use energy dispersive X-ray analysis (EDS or
EDX) to find out what elements are present at the
surface
EDS output from X-rays
Amount of
packets
1050
900
CKa
1200
Characteristic carbon peak
006
Characteristic oxygen peak
Characteristic chlorine peak
ClKa
600
OKa
Counts
750
450
300
150
0
0.00
1.00
2.00
3.00
4.00
5.00
keV
Energy of packets
in thousands of electron volts
6.00
7.00
8.00
9.00
10.00
Lets start by looking at relatively modern
coins
1958 sixpence
Equivalent to 5 cents
1 cent coin: Australian
5 cent coin: Australian
What is a sixpence made of?
Above:
secondary
electron image
of coin shows
surface detail
Left:
backscattered
electron image
shows average
atomic number
[by contrast]
Why is a sixpence
silver to look at?
020
4000
020
The spectrum (left)
shows the X-ray
analysis of a
sixpence (on blue
cross)
ClKa ClKb
AgLb AgLa
3600
2800
1600
1200
800
CuLa
2000
CKa
Counts
2400
1.0
1.0 mm
mm
CuKa
3200
Note the silver
peaks
400
0
0.00
1.00
2.00
3.00
4.00
5.00
keV
6.00
7.00
8.00
9.00
What else is
present?
Standardless Quantitative Analysis [phi-rho-z method]
Element
keV (energy)
Mass %
Error %
Atomic %
C
0.277
6.02
0.11
35.63
Cl
2.621
0.36
0.12
0.73
Cu
8.040
4.14
0.82
4.64
Ag
2.983
89.48
0.33
59.00
Total
100.00
100.00
If the mass composition is the same as the
surface then there is about 89 mass % Ag
A sixpence looks silver because it contains
a lot of silver
The coin weighs 2.8 grams
What would it be worth in today’s metals
market?
At $480 AUS/ kg for silver the coin is
worth about $1.20 AUD
An Australian 5 cent coin also looks silver
but does it contain silver?
Above:
backscattered electron
image
What are the dark spots?
022
CuKa
3600
2000
1600
1200
800
CKa
Counts
2400
NiKb
CuKb
2800
The spectrum
(left) shows the
X-ray analysis of
a 5 cent coin
NiKa
OKa
CuLa NiLa
3200
Note the copper
and nickel peaks
400
0
0.00
1.00
2.00
3.00
4.00
5.00
6.00
7.00
8.00
9.00
keV
The dark spots on the previous image will be
element with a low atomic number such as carbon
– they may be an area of oxide or dirt.
On the area we tested we can calculate
composition
Standardless Quantitative Analysis [phi-rho-z method]
Element
keV
(energy)
C
0.277
O
Error %
Atomic %
8.60
0.11
31.98
0.525
1.13
0.09
3.17
Ni
7.471
23.96
0.25
18.24
Cu
8.040
66.30
0.33
46.62
Total
Mass
%
100.00
100.00
If the whole composition is the same as
the surface then there is about 66 mass
% Cu and about 24 mass % Ni
Why does the coin look silver?
Nickel mixed with copper produces a
silver look to the coin
This example 5 cent coin weighs 2.9
grams
By looking up the current value of
copper and nickel the value of the
metal can be calculated
A copper 1 cent coin looks brown
What is it made of?
1000
002
002
CuLa
800
700
1.0
1.0 mm
mm
300
Is anything else
present?
OKa
CKa
500
400
ClKa ClKb
600
Counts
Note the copper
peaks on the
right
CuKa
900
200
100
0
0.00
1.00
2.00
3.00
4.00
5.00
keV
6.00
7.00
8.00
9.00
10.00
On the area we tested we can calculate
composition
Standardless Quantitative Analysis [phi-rho-z method]
Element
keV (energy)
C
0.277
O
Error %
Atomic %
6.44
0.24
24.65
0.525
3.42
0.20
9.83
Cl
2.621
0.55
0.13
0.72
Cu
8.040
89.59
0.69
64.81
Total
Mass %
100.00
100.00
The 1 cent coin has about 90 mass % Cu
so it is brown because of the copper
Our copper coin weighs 2.7 grams
With copper selling at about $8.94 AUD/ kg
What is the metal in the coin worth?
About 2 cents!
What can Roman coins tell us?
Roman coin
From Alexandria
Constantine I
Note: never clean an old coin – it removes
value! This one has a patina of green corrosion
Roman coin from Alexandria (ear region of
Constantine I) seen using backscattered
electrons
Ear region
What are the white blobs in the coin on the
previous image?
What is the surrounding material?
CuKb
CuKa
FeKb
300
FeKa
450
SnLa
600
FeLa
Counts
750
SiKa
PbMa
900
016
AlKa
1050
016
CKa
OKa
CuLa
1200
150
0
1.0
1.0 mm
mm
0.00
1.00
2.00
3.00
4.00
5.00
6.00
7.00
keV
8.00
The Roman coin contains copper, lead,
tin, and may have some dirt (clay: Al,
Si and Fe) on the surface. There is
oxide present too.
We can say the basic coin is bronze
9.00
10.00
600
CuKb
900
FeLa
1200
CKa
Counts
1500
OKa
1800
CuKa
017
FeKa
2100
FeKb
017
CuLa
AlKa
SiKa
SKa
ClKb AgLa ClKa
AgLb
CaKa
2400
300
0
1.0
1.0 mm
mm1.00
0.00
2.00
3.00
4.00
5.00
6.00
7.00
8.00
keV
The white blobs are about 50 mass % silver
What does this mean?
Was the ore the coin was made from
contaminated with silver? Or was the crucible
in which the copper was melted the source of
the silver?
9.00
Bronze coins are thought to have remained
near where they were made because they
were not particularly valuable and therefore
not carried far
So, their metal composition reflects the
mines and local manufacturers techniques
Silver is not found in the bronze coin found
in London [analysis not shown here], only in
the one from Egypt (Alexandria)
View of both sides of Roman coin from
London